The effects of ambient conditions (25oC), increasing sonication time (0–150 min), temperature (30oC and 60oC), Ferrous ions (Fe+2) (2 mg/l, 8 mg/l and 20 mg/l) and Ferric ions (Fe+3) (10 mg/l, 20 mg/l and 50 mg/l) concentrations on the removal of polycyclic aromatic hydrocarbon (PAH) and destruction of toxicity in a petrochemical industry wastewater (PCI ww) in Izmir (Turkey) were investigated. The maximum PAH removals were 80.2%, 91%, 98.56% and 96.76% at 25oC, at 60oC, at Fe+2 =20 mg/l and at Fe+3 =50 mg/l, respectively, after 150 min sonication time. Sonication alone provides PAH removals varying between 90.11% and 96.90% without Fe+2 and Fe+3 at 30oC and 60oC after 150 min sonication time. The Daphnia magna acute toxicity decreased significantly from EC=342.6 mg/l to EC=50 mg/l, to EC=32 mg/l and to EC=15 mg/l, respectively, as the temperature, the Fe+2 and Fe+3 concentrations were increased. The matrix for the maximum Daphnia magna acute toxicity removals (100%) was 80% Daphnia magna acute toxicity yield for Fe+2=20 mg/l and 70% Daphnia magna acute toxicity yield for Fe+3=10 mg/l, respectively, at 60oC after 150 min sonication time, respectively. The Daphnia magna acute toxicity measurement procedure was successfully applied to seventeen PAHs removals during sonication process in the PCI ww with the addition of different Fe+2 and Fe+3 ions concentrations, respectively. The PAH sonodegradation appeared to be Pseudo first order in PAHs naphthalene (NAP), acenaphthylene (ACL), phenanthrene (PHE), pyrene (PY) and benz[b] fluoranthene (BbF) (k=0.026 1/min, k=0.024 1/min, k=0.017 1/min, k=0.015 1/min and k=0.011 1/min, respectively). The main mechanism of PAH sonodegradation appears to be pyrolysis.

PAHs are listed as US-EPA and EU priority pollutants, and their concentrations, therefore, need to be controlled in treated wastewater effluents [1, 2]. Due to their toxic, mutagenic and carcinogenic properties the US-EPA classifies 16 of these PAHs as priority pollutants [1, 2]. Recent studies have shown that sonication may be a useful tool for degrading the aqueous pollutants [3-6]. The sonication process is capable of effectively degrading target compounds including chlorophenols, chloroaromatics and PAHs present in dilute solutions, typically in the micro and nano ranges. The process does not require the use of additional chemicals commonly employed in several oxidation processes, thus again reducing costs. David [6] found that naphthalene (NAP), phenanthrene (PHE), anthracene (ANT) and pyrene (PY) removal efficiencies varied between 93% and 95%, after a sonication time of 90 min in a sonicator with a power of 400 W and a frequency of 20 kHz. Psillakis et al. [7] reported a 99% removal efficiency for 0.01 mg/l of acenaphthalene (ACT), PHE and NAP at a power of 300W and frequency of 24 kHz. Benabdallah El-Hadj et al. [5] found 57% NAP, 40% PY and 45% total COD removal efficiencies in a sonicator at 70 W sonication power and at 20 kHz sonication frequency. Taylor et al. [8] investigated the sonication of PAHs, namely ANT, PHE and PY. 46%, 20% and 50% removal efficiencies, respectively, were found at 600 W and at 20 kHz. Laughrey et al. [9] investigated the effects of DO, air on the sonication of PHE, PY and ANT. They found removals of these PAHs as high as 80%–90% as the DO concentration, air and N2(g) purges were increased from 1 mg/l to 5 mg/l and from 2 ml/min to 4 ml/min and from 3 ml/min to 6 ml/min, respectively.

When sonolysis of water occurs, it leads to the formation of the non-specific oxidative species OH●. The ultrasonic degradation of hydrophobic organics such as PAHs can occur when they penetrate to the surrounding of the hot heart of the cavitation bubble being pyrolyzed, burnt and/or ionized in the plasma core [10, 11]. The literature data concerning the sonodegradation of PAHs is scarce and the results are contradictory. Two mechanisms have been proposed to account for sonolytic degradation: (i) oxidation by OH● [8, 9] and (ii) pyrolytic decomposition [7].

In Izmir, Turkey, petrochemical plant wastewaters are treated with conventional activated sludge systems. Since such systems are unable to completely remove the main PAHs present (ca. 17) these are released into receiving bodies. Although some studies aimed at increasing the degradation of some PAHs (NAP, PHE, ANT, PY and ACT) with sonication have appeared, these have been limited to only a few of those generally present (3–5) [5, 7, 12, 13]. No study was found investigating the effects of operational conditions such as sonication time, temperature, Fe+2 and Fe+3 on the sonication of a PCI ww. Furthermore, the effects of the operational conditions on the removal of acute toxicity has not been determined for a PCI ww. Thus, in this study our aim was to determine the effects of ambient conditions, increasing sonication time (0 min, 60 min, 120 min and 150 min), increasing sonication temperatures (25oC, 30oC and 60oC), increasing Fe+2 ion (2 mg/l, 8 mg/l and 20 mg/l) and Fe+3 ion (10 mg/l, 20 mg/l and 50 mg/l) concentrations on the sonodegradation of seventeen PAHs. The effects of these operational conditions on the acute toxicity of Daphnia magna microorganisms were determined. Furthermore, the reaction kinetics of five representative PAHs and the mechanism of PAH sonodegradation were investigated.

Sonicator and Operational Conditions 

A BANDELIN Electronic RK510 H sonicator was used for sonication of the PCI ww samples. The wastewater was not pre-treated before sonication since the solids was disentegrated through sonication. Glass serum bottles in a glass reactor were filled to a volume of 100 ml with PCI ww after the dosing of oxygen (O2) and hydrogen peroxide (H2O2). They were then closed with teflon coated stoppers for the measurement of volatile compounds (evaporation) of the petrochemical wastewater. The evaporation losses of PAHs were estimated to be 0.01% in the reactor and therefore, assumed to be negligible. The serum bottles were filled with 0.1 ml methanol (CH3OH) in order to prevent adsorption on the walls of the bottles and minimize evaporation. The temperature in the sonicator was monitored continuously and was maintained constant at 30oC and at 60oC. For ambient conditions the sonicator was not heated – it was used at 25oC. All experiments were in batch mode using an ultrasonic transducer (horn type), which has an active acoustical vibration area of 19.6 cm2, and a maximum input power of 650 W. Four sonication intensities (16 W/m2, 37 W/m2, 23.02 W/m2 and 51.75 W/m2) were chosen to identify the optimum intensity for maximum PAH removal. Samples were taken after 60 min, 120 min and 150 min of sonication and were kept for a maximum of 15 min in a refrigerator at a temperature of +4oC until the sonication experiments were begun. Air enriched with O2(g) was provided to the samples before sonication. Dissolved oxygen (DO) was sparged into the liquid samples with a pump under a pressure of 0.5 atm for 10 min at a flow rate of 5 ml/min (monitored by a rotameter), and then stopped. H2O2 solutions were slurried in the reaction mixture with a pressured pump 20 min prior to sonication at a flow rate of 100 ml/min and then stopped. 

Analytical Methods 

For PAHs and some metabolites (phenanthrenediol, naphthalene and p-hydroxybenzoic acid by-products and fluorene) analyses the samples were first filtered through a glass fiber filter (47-mm diameter) to collect the particle-phase in series with a resin column (∼10 g XAD-2) and to collect dissolved-phase polybrominated diphenyl ethers. Resin and water filters were ultrasonically extracted for 60 min with a mixture of 1/1 acetone/hexane. All extracts were analyzed for seventeen PAHs including naphthalene (NAP), acenaphthylene (ACL), acenaphthene (ACT), fluorene (FLN), phenanthrene (PHE), anthracene (ANT), carbazole (CRB), fluoranthene (FL), pyrene (PY), benz[a]anthracene (BaA), chrysene (CHR), benz[b] fluoranthene (BbF), benz[k]fluoranthene (BkF), benz[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (IcdP), dibenz[a,h]anthracene (DahA), and benzo[g,h,i]perylene (BghiP), respectively, gas chromatographically (Agilent 6890N GC) equipped with a mass selective detector (Agilent 5973 inert MSD). A capillary column (HP5-MS, 30 m, 0.25 mm, 0.25 m) was used. The initial oven temperature was kept at 50oC for 1 min, then raised to 200oC at 25oC/min and from 200oC to 300oC at 8oC/min, and then maintained for 5.5 min. High purity He(g) was used as the carrier gas at constant flow mode (1.5 ml/min, 45 cm/s linear velocity). PAHs and their metabolites were identified on the basis of their retention times, target and qualifier ions and were quantified using the internal standard calibration procedure. The Phenanthrenediol analysis was performed using a high-pressure liquid chromatography (HPLC) (Agilent-1100) with a method developed by Lindsey and Tarr [14]. The chromatographic conditions for the phenanthrenediol determination were as follows: C-18 reverse phase HPLC column (Ace 5C18; 25 cm × 4.6 mm, 5 m, mobile phase: 50/50 (v/v) methanol/organic-free reagent water). The NAP, p-hydroxybenzoic acid by-products and FLN were measured in the aforementioned HPLC by using C-8 column (Ace 8; 15 cm × 2.6 mm, 3 m, mobile phase: 70/30 (v/v) methanol/organic-free reagent water). The CH4, CO2 and H2S gas analysis was performed following Standard Methods [15]. pH, temperature, oxidation-reduction potential (ORP), COD and TOC concentrations were monitored following the Standard Methods 2550, 2580, 5220 D and 5310 [15]. H2O2 was quantified with a colorimetric method following the Standard Methods 3550 [15]. DO, pH were measured in a WTW dissolved oxygen meter and pH meter the ORP was measured using a WTW redox meter. TSS, TVSS, inorganic nitrogen compounds, Total-P, oil and SO4 were monitored following the Standard Methods [15]. 

To test toxicity 24 h old Daphnia magna were used as described in Standard Methods [15]. After preparing the test solution, experiments were carried out using 5 or 10 daphnids introduced into the test vessels. These vessels had 100 ml of effective volume at 7.0–8.0 pH, providing a minimum DO concentration of 6 mg/l at an ambient temperature of 20–25oC. Young Daphnia magna were used in the test ( ≤ 24 h old). A 24 h exposure is generally accepted as standard for a Daphnia magna acute toxicity test. The results were expressed as mortality percentage of the Daphnids. Immobile animals were reported as dead Daphnids. All experiments were carried out three times and the results given as the means of triplicate samplings. Individual PAH concentrations are given as the mean with standard deviation (SD) values.

Multiple regression analysis between y and x variables was performed using the Excell in Windows. The linear correlation was assessed with R2. The significance of the correlations between data was determined using the ANOVA Test Statistics.

Raw Wastewater
Characterization of raw PCI ww taken from the influent of the aeration unit of a PCI ww treatment plant was performed. The results are given as the mean value of triplicate samplings (Table 1). 

Table 1: Characterization of raw PCI ww taken from the influent of the aeration unit of a PCI ww treatment plant (n = 3, mean values ± SD).

Preliminary studies showed that high ultrasound frequencies of 80 kHz and 150 kHz did not increase the results of the parameters studied. Therefore, they were studied at a sonication frequency of 35 kHz. Increasing the sonication frequency decrease the Therefore, they were studied at a sonication frequency of 35 kHz. Increasing the sonication frequency decrease the number of free radicals, therefore they did not escape from the bubbles and did not migrate [16]. Among the sonication intensities applied to the sonication process (16 W/m2, 37 W/m2, 23 W/m2 and 51.8 W/m2) in this study the most effective sonication intensity was found to be 51.8 W/m2 [16]. The degradation of PAHs increased with increasing applied power. Therefore, in this study the power of the sonicator was adjusted to be 640 W. As the power increased, the number of collapsing cavities also increased, thus leading to enhanced degradation rates, as reported by Psillakis et al. [7] and Papadaki et al. [17]. It has been shown that increasing the ultrasonic intensity improves the degradation rate of organic compounds [17]. Furthermore, collapse of bubbles in the reaction cell of the sonicator occur more rapidly and the number of cavitation bubbles increases. Thus, produces higher concentration of OH● radicals at higher ultrasonic intensities. These OH● radicals react with PAHs in the solution. Therefore, the increased degradation of PAHs noted on increasing the ultrasonic power arises from the enhancement of radical yields.

Effect of Increasing Sonication Time on the PAH Removal Efficiencies in Ambient Conditions 
Raw PCI ww samples were sonicated at an ambient temperature of 25oC at increasing sonication time (60 min, 120 min and 150 min). The results of the study showed that as the sonication time was elevated the total PAH removals increased. 54.9%, 61.3% and 79.6% maximum total PAH removal efficiencies were observed after 60 min, 120 min and 150 min sonication time, respectively, at an influent total PAH concentration of 1380 mg/l (Figure 1). 

Figure 1: Effect of sonication time on the total PAH removals at ambient conditions (T=25oC) (n = 3, mean values ± SD)

Much of the PAH decomposition was accomplished in the initial 60 min of sonication and the efficiency of this decomposition increased approximately 10% and 25% by increasing the time from 60 min to 120 min and 150 min sonication time, respectively. The effect of sonication time on the total PAH removal was significant (p < 0>

Figure 2: Effect of increasing sonication time on PAH removal efficiencies in ambient temperature (T=25oC) (n = 3, mean values ± SD).

Although the total PAH removals increased at increasing sonication times among the PAHs studied it was found that PHE, BghiP and PY concentrations decreased as the sonication time increased from 60 min to 120 min while the concentration of these PAHs increased after 150 min. With the increase of sonication time, the amount of naphthalene and p-hydroxybenzoic acid by-products and fluorene first increased and then decreased, after 60 min and 120 min throughout sonication of PHE, respectively, suggesting formation and decomposition reactions of these by-products (data not shown). Since the percentage of PHE remaining decreased for 60 min and 120 min and then increased after 150 min with increasing sonication time we suspected that the increase of PHE with longer sonication may be due to the formation of PHE from by-products such as fluorene. A decrease of the percentange remaining PHE was expected at longer sonication times due to high temperature and radical reactions from cavitation. A radical mechanism proposed by David [6] showed PHE formation from pyrolysis of 9,9-dimethylfluorene at high temperatures by a free radical ring expansion process. Thus, fluorene formed during the sonication of PHE may be attacked by methyl radicals from hexane and acetone dissociation to regenerate PHE. In addition, different types of radicals (e.g., methyl, ethyl) were produced from the dissociation of solvents. Cyclization reactions of these radicals with methyl or ethyl naphthalene may also contribute to the reformation of phenanthrene. Wu and Ondruschka [18] also reported NAP and benzene formation during PHE pyrolysis.

The PAH removals found in our study were high in comparison to the study performed by David [6]. They found 74%, 72% and 76% PHE, NAP and ACL degradation rates, respectively, at 40oC after 150 min sonication time [19]. Similarly, in a study performed by Litlee et al. [20] it was found that 0.6 mg/l PHE was removed with low yields (32%) at 22oC after 20 min sonication time. However, increasing the sonication time to 135 min led to about 56% removal in a sonicator with at 30 kHz and at 320 W.

Raw PCI ww samples were sonicated in a sonicator at 30oC and 60oC for 0 min, 60 min, 120 and 150 min sonication time. No influence of temperature increase on PAH removals at ambient temperatures from 25oC to 30oC and 60oC after 60 min sonication time was observed (Figure 3). 

Figure 3: Effect of increasing temperature on the total PAH removal efficiencies at increasing sonication times (n = 3, mean values ± SD).

As the temperature was increased from 25oC to 30oC the total PAH removal did not change after 60 min sonication time and remained between 46% and 50% compared to the control (the sonicator was operated under ambient temperature of 25oC). The total PAH removal efficiency raised from 58.1% to 78.3% at 60oC after 120 min sonication time. Temperature increase from 30oC to 60oC elevated the total PAH removal up to 88%–91%, respectively, after 150 min sonication time. The removal efficiencies for PAHs with 3, 4, 5 and 6 benzene rings were > 87% at 60oC after 150 min sonication time (Table 2). 

                                                                  
Table 2: Influent and effluent PAH concentrations and maximum PAH removal efficiencies in the sonication experiments at DO=6 mg/l and 30oC after 150 min sonication time (n = 3, mean values ± SD).

The individual PAH removal efficiencies versus increasing temperature are given in Figure 4 for 60 min, 120 min and 150 min sonication time, at 60oC

Figure 4: Maximum PAH removal efficiencies at increasing sonication times at 60oC (n = 3, mean values ± SD).

The maximum removals for PAHs with one ring (NAP 98.2%, and ACL 97.2%), three rings (CRB 97.9%, CHR 95.1) four and five rings (BaP 97.3%, IcdP 98.2% and BghiP 96.3%) were determined at 60oC after 150 min sonication time. Among the PAHs studied, only in the case of PY temperature increase did not influence its removal. The yield of PY decreased slightly on temperature rose from 120oC to 150oC while the removals of PHE, BghiP and the rest of the PAHs elevated. The slight decrease in degradation rate observed for PY may be due to the increased solution temperature. For PY, an elevated solution temperature might imply a slightly higher adsorption on the air–water interface and an improved diffusivity. These factors act to affect the slight accumulation of PY on the interface in different ways. As the temperature increased, the raised diffusivity may contribute tomore available PY at the subsurface for adsorption. Thus, a slight increment in removal efficiency was observed from 25oC to 60oC. The decrease in removal efficiency at 150oC may be due to less favorable adsorption resulting in reduced accumulation on the interface.

Although, the effects of raising temperature on the sonolytic removal efficiencies were also examined for all PAHs, in this section only PHE and BghiP PAHs are discussed. The removal yields of PHE and BghiP improved with increasing temperature. For partitioning into the bubble, the elevated solution temperature will allow PHE and BghiP molecules to more easily enter the cavitation bubble (i.e., increase diffusivity). At higher temperatures this effect will be enhanced and this may be the cause of the improve in removal rates for PHE and BghiP at 150oC.

Different suggestions were presented to explain the effect of temperature on the sonochemical degradation of PAHs since it is a relatively complex issue closely related to the properties and reaction conditions of each specific system in question. As the temperature elevates the collapse temperature of the cavitation bubble should decrease [6, 9]. However, other studies have shown that after an initial increase in solution temperature the rate of reaction increases leading to a greater fraction of volatile compounds partitioning into the cavity. A further increase in solution temperature leads to a decrease in the rate of reaction [7]. Therefore, it is not surprising that several investigators have reported contradictory findings regarding the temperature effect. In certain reaction systems for instance, the net effect of an increment in T0 and consequently Tmax, is an increase in degradation rates. This occurs up to the point at which the cushioning effect of the vapour begins to dominate the system and further increases in liquid temperature result in reduced reaction rates. The fact that removal decreases with raising liquid temperature is believed to be associated with the effect of temperature on both the bubble formation energy threshold and the intensity of bubble implosion. The maximum temperature (Tmax) obtained during the bubble collapse is given as follows (Equation 1):

      (1)                                                                                                                                     

where; T0 is the liquid bulk temperature, P0 is the vapour pressure of the solution, P is the liquid pressure during the collapse and is the specific heat ratio (i.e. the ratio of constant pressure to constant volume heat capacities). Increased temperatures are likely to facilitate bubble formation due to an increase of the equilibrium vapour pressure; nevertheless, this beneficial effect is compensated by the fact that bubbles contain more vapour which cushions bubble implosion and consequently reduces Tmax. In addition to this, increased temperatures are likely to favour degassing of the liquid phase, thus reducing the number of gas nuclei available for bubble formation [7]. It was observed that the PAHs with multiple benzene rings were also degradable with high yields, even though some studies demonstrated that sonication is not effective for PAHs with a large number of benzene rings [7]. The PAHs removal yields obtained in our study are high in comparison to the removal performances of PAHs by the studies given below. In the study by Laughrey et al. [9] 77% PAH removal efficiency was observed for the sonochemical degradation of a PAHs mixture (50 mg/l NAP, 55 mg/l ACL and 52 mg/l PHE) in water after 120 min sonication time, at 40oC, at 150 W and at 24 kHz, respectively. Benabdallah El-Hadj et al. [5] found 31%–34% and 44%–50% PAH removal effiencies in mesophilic (35oC) and thermophilic (55oC) conditions for NAP and PY by a sonicator, at 20 kHz and at 70 W, after 110 min sonication time, before anaerobic digestion.

2 mg/l, 8 mg/l and 20 mg/l Fe+2 ions (from FeCl2.4H2O) were added to the PCI ww before the sonication experiments. 83.27%, 92.62% and 95.12% total PAHs removals were obtained in 2 mg/l, 8 mg/l and 20 mg/l Fe+2, respectively, after 150 min sonication time, at pH=7.0 and at 30oC (Figure 5a). No significant increase in PAHs yields was obtained from 8 mg/l to 20 mg/l Fe+2 after 150 min sonication time, at pH=7.0 and at 30oC, compared to the control (without Fe+2 while E=90.11% for total PAHs at pH=7.0 and at 30oC). A significant linear correlation between total PAHs yields and increasing sonication time was not observed (R2=0.30, F=0.28, p=0.01) (Figure 5a).

                                                                                     
Figure 5: Effect of increasing Fe+2 concentrations on the total PAHs removal efficiencies in PCI ww at (a) 30oC and (b) 60oC versus increasing sonication times (at 640 W and at 35 kHz).

86.28%, 95.27% and 98.56% total PAHs yields were observed in 2 mg/l, 8 mg/l and 20 mg/l Fe+2, respectively, after 150 min sonication time, at pH=7.0 and at 60oC (Figure 5b). No significant increase in total PAHs yields were obtained by increasing the Fe+2 concentrations compared to the control after 120 min and 150 min sonication time, at pH=7.0 and at 

60oC. Sonication alone provided 96.90% total PAHs yield after 150 min sonication time, at pH=7.0 and at 60oC. The maximum total PAHs removal efficiency was 98.56

Increasing Fe+3 concentrations (10 mg/l, 20 mg/l and 50 mg/l) were added to the PCI ww before sonication process. 82.92%, 91.72% and 93.58% total PAHs removals were measured in 10 mg/l, 20 mg/l and 50 mg/l Fe+3, respectively, after 150 min sonication time, at pH=7.0 and at 30oC (Figure 7a). An increase of 14.66%-22.28% and 16.21%-20.55% in total PAHs yields were measured for after 60 min and 120 min sonication time, compared to the control (without Fe+3) at pH=7.0 and at 30oC. Although, a correlation between PAHs removal efficiencies and Fe+3 concentrations were obtained this relationship was not significant (R2=0.76, F=2.56, p=0.01). Control provided 90.11% total PAHs yield after 150 min sonication time, at pH=7.0 and at 30oC (Figure 7a).'

Figure 7: Effect of increasing Fe+3 concentrations on the total PAHs removal efficiencies in PCI ww at (a) 30oC and (b) 60oC versus increasing sonication times (at 640 W and at 35 kHz).

84.61%, 93% and 96.76% total PAHs yields were observed in 10 mg/l, 20 mg/l and 50 mg/l Fe+3, respectively, after 150 min sonication time, at pH=7.0 and at 60oC (Figure 7b). The contribution of increasing Fe+3 on the total PAHs removal was only 7.06%-15.41% and 1.25%-5.04% compared to the control after 60 min and 120 min sonication time, at pH=7.0 and at 60oC. The maximum total PAHs removal efficiency was 96.76

The sonic degradation of 17 PAHs in the raw PCI ww was found to be pseudo first order with respect to PAH concentrations at a frequency of 35 kHz and 60oC (Equation 5).

       (5)                                                                                                                           

The degradation rates for all PAHs were deduced from the slopes of the curves given by (Equation 6);

           (6)                                                                                                                         

where, k[PAH] is the rate constant at 35 kHz, [PAH]0 the initial PAH concentration and [PAH]t its value at time t. In this study, the rate constants of sonodegradation are given for only five PAHs. These rate constants are tabulated in Table 5. 

Table 5: Calculated steady-state [HO•]ss concentrations for five PAHs, comparison of PAH oxidation rates of OH● and experimental PAH removal rates after 150 min sonication time, at 35 kHz and at 60oC (n = 3, mean values).

The pseudo first order rate constants ranged between k=0.011 1/min and k=0.026 1/min.The pseudo first order kinetic rate constants obtained in this study agree with the literature data of low frequencies (20 kHz and 32 kHz) [5, 8, 12]. The biodegradation rate constants of the PAHs depend on their properties, such as the benzene ring numbers, the vapour pressure, the water solubility and Henry’s law constant, as reported by David [6]. The biodegradation rate constants increased as the vapour pressure, the water solubility and the Henry’s law constants increased and the number of benzene rings of PAHs decreased. The most hydrophobic PAHs, with four and five benzene rings (BkF, BaP, IcdP, DahA, BghiP) (low water solubility and Henry’s law constant) have the lowest degradation constants compared to one, two and three ring PAHs (NAP, ACL, ACT, FLN, PHE, ANT) with high water solubility and Henry’s law constants. The data obtained in this study are in good agreement with the rate constants obtained by David [6] and Wu and Ondruschka [18]. The pseudo first order rate constants decreased from PAHs with one benzene ring to PAHs with five rings at low sonication times such as 60 min (Table 5). However, the untreated percentages of PAHs with four and five rings at a sonication time of 150 min reached the same levels as PAHs with low ring numbers after 150 min sonication time (Figure 8). Therefore, the removal efficiencies for all PAHs were > 95

The Daphnia magna test is accepted as an acute toxicity test. Toxicity was estimated in terms of EC50, defined as the concentration of the toxicant causing 50% reduction in activity of the water flea. Table 6 shows the acute toxicity test results obtained from the Daphnia magna test through sonication with increasing temperatures, Fe+2and Fe+3 concentrations. 

The test samples containing an initial total PAH concentration of 1380 mg/l were diluted at 1/1, 1/2, 1/8, 1/16 and 1/24 ratios after sonication experiments. 10 young Daphnids ( < 24 xss=removed>

Table 6: Effect of sonication on the Daphnia magna acute toxicity (EC50) removal efficiencies under different operational conditions [T(oC)= 25–60oC; T(min)= 60–150 min; Fe+2: 2–20 mg/l; Fe+3: 10–50 mg/l, n = 3, mean values].

The EC50 values decreased to EC30 to EC20 and to EC10 after 60 min, 120 min and 150 min sonication time, respectively, in Fe+2=8 mg/l at 60oC (Table 6; SET 3). The EC30,the EC20 and the EC10 values were measured as 310 mg/l, 150 mg/l and 47 mg/l, respectively, in Fe+2=8 mg/l at 60oC. The toxicity removal efficiencies were 40%, 60% and 80

The results of this study show that PAHs in PCI ww could be treated efficiently with low-frequency sonication. Although, the degradation efficiency of PAHs in PCI ww was affected by the time, temperature, Fe+2 and Fe+3 sonication alone could provide 90.11%-96.90% PAH removals at 30oC and 60oC after 150 min sonication time. The optimum operational conditions for maximum PAH removals at 25oC and 60oC were Fe+2=20 mg/l and Fe+3=50 mg/l, respectively, after 150 min sonication time. All the PAHs were removed with treatment efficiencies above 87% (up to 99%) after 150 min sonication time, at 60oC.

The matrix for the maximum Daphnia magna acute toxicity removals (100%) was 80% Daphnia magna acute toxicity yield for Fe+2=20 mg/l and 70% Daphnia magna acute toxicity yield for Fe+3=10 mg/l, respectively, at 60oC after 150 min sonication time, respectively. The Daphnia magna acute toxicity measurement procedure was successfully applied to seventeen PAHs removals during sonication process in the PCI ww with the addition of different Ferrous ions and Ferric ions concentrations, respectively. 

The PAH sonodegradation appeared to be Pseudo first-order rate constants in PAHs  NAP, ACL, PHE, PY and BbF (k = 0.026  1/min, k=0.024  1/min, k=0.017  1/min, k=0.015  1/min and k=0.011  1/min, respectively). The main PAH degradation mechanism during ultrasonic irradiation is pyrolysis. 

This research study was undertaken in the Environmental Microbiology Laboratories at Dokuz Eylül University Engineering Faculty Environmental Engineering Department, Izmir, Turkey. The authors would like to thank this body for providing financial support.